Re: [PATCH v2] KVM: halt-polling: poll if emulated lapic timer will fire soon

[David Matlack]

On Sun, May 22, 2016 at 6:26 PM, Yang Zhang <yang.zhang.wz@xxxxxxxxx> wrote:
> On 2016/5/21 2:37, David Matlack wrote:
>>
>> It's not obvious to me why polling for a timer interrupt would improve
>> context switch latency. Can you explain a bit more?
>
>
> We have a workload which using high resolution timer(less than 1ms) inside
> guest. It rely on the timer to wakeup itself. Sometimes the timer is
> expected to fired just after the VCPU is blocked due to execute halt
> instruction. But the thread who is running in the CPU will turn off the
> hardware interrupt for long time due to disk access. This will cause the
> timer interrupt been blocked until the interrupt is re-open.

Does this happen on the idle thread (swapper)? If not, halt-polling
may not help; it only polls if there are no other runnable threads.

> For optimization, we let VCPU to poll for a while if the next timer will
> arrive soon before schedule out. And the result shows good when running
> several workloads inside guest.

Thanks for the explanation, I appreciate it.

>
> --
> best regards
> yang